Heretofore, there has been proposed as a form of an image display apparatus a liquid crystal panel or a projection display for applying a light beam modulated by a spatial modulator such as a DMD (Digital Micromirror Device) or the like to display an image on a screen. Such a projection display has employed a lamp such as a metal halide lamp, a halogen lamp, or a xenon lamp as a light source.
However, in the case where those lamps are used as a light source, there is a problem that they have a short life and need complex maintenance. Another problem is that an optical system for extracting three primary colors from white light emitted from the lamp is complicated, and a color reproduction range is limited, resulting in reduced light utilization efficiency.
For solving the above problems, attempts have been made to use a laser beam source such as a semiconductor laser or the like as the light source of a projection display. The laser beam source has many advantages in that it has a longer service life than lamps, an excellent directivity for efficient utilization of radiated light, thus providing a high light utilization efficiency, and has a wide color reproduction range because of its monochromatic nature.
In recent years, a display employing an active-drive grating (diffraction grating) fabricated by the micromachining technology has been developed and drawing attention. The diffraction grating device that is employed is called “grating light valve” (hereinafter referred to as “GLV”). The display has such features that it can display seamless, clear, and bright images compared with the conventional display using a spatial modulator, can be manufactured inexpensively by the micromachining technology, and can operate at a high speed.
It has been considered to use a laser beam source such as a semiconductor laser or the like as a light source for illuminating a GLV.
However, if a laser beam source is used as the light source of an image display apparatus such as a projection display or the like as described above, then speckles (or speckle noise) are generated, degrading the image quality.
The speckle noise is a phenomenon caused when coherent light emitted in uniform phase from a laser beam source is dispersed by a random phase surface (object surface), and disturbed wavefronts propagated from adjacent regions on the object surface interfere with each other on an observation surface. The speckle noise appears as a granular intensity distribution on the observation surface.
If such speckle noise occurs between the screen as the object surface of the projection display with the laser beam source and the observer's eye (retina) as the observation surface, then the observer recognizes the speckle noise as image degradation. It is therefore important to reduce speckle noise for image display apparatus such as a projection display with a laser beam source.
If an image display apparatus such as a projection display or the like employs a laser beam of low spatial coherence, it may be able to produce a two-dimensional image having reduced speckle noise. However, since a laser beam of low spatial coherence tends to cause noise and the beam shape and intensity distribution thereof are liable to vary and hence unstable, it is not suitable for use as a light source in an image display apparatus such as a projection display or the like.
A high-output visible laser beam whose beam shape and intensity distribution are stable is generally of high spatial coherence, and tends to cause the above problem of speckle noise.
It is therefore an object of the present invention to reduce speckle noise to lower image degradations in an image display apparatus such as a projection display or the like.